1. Field of the Invention
The present invention relates to communication between a base station and terminal equipments in a mobile communication system.
2. Description of the Related Art
On the air interface between a base station and terminal equipments in a Long Term Evolution (LTE) mobile communication system, control signals and data signals are transmitted over various channels according to the kinds of the signals. For example, a random access channel (RACH) is mapped to a physical random access channel (PRACH) and a preamble and a message are transmitted over the RACH. Over a physical uplink shared channel (PUSCH), an upstream data signal is transmitted. Over a physical uplink control channel (PUCCH), an upstream control signal is transmitted. Each of reference signals contained in the frames on the PUSCH and the PUCCH and the preamble on the RACH is a signal of a predetermined pattern.
The base station detects the reference signal on the PUSCH or the PUCCH to enable, for example, compensation for attenuation of the upstream data signal and the upstream control signal from each terminal equipment. Also, the base station can recognize access from each terminal equipment by detecting the preamble on the RACH.
In the LTE mobile communication system, code division multiplexing (CDM) is performed for multiplexing transmission of reference signals from a plurality of terminal equipments in the same bandwidth. In CDM, a cyclic shift specific to each terminal equipments is used to enable the plurality of terminal equipments to share the same Zadoff-Chu sequence (ZC sequence) while maintaining orthogonality.
In ordinary cases, terminal equipment performs signal processing including Fourier transform and inverse Fourier transform after effecting a cyclic shift of a predetermined ZC sequence, and transmits a signal thereby obtained (see R1-060373: Comparison of Proposed Uplink Pilot Structures For SC-FFOMA, TEXAS INSTRUMENTS, 3GPP TSG RAN WG1#44 Denver, Co, Feb. 13-17, 2006).
FIG. 1 is a block diagram showing an example of a configuration for terminal equipment made by considering transmission of the reference signal. In the example shown in FIG. 1, the terminal equipment effects a cyclic shift specific to the terminal equipment on a predetermined ZC sequence, and thereafter converts the ZC sequence into a frequency region by discrete Fourier transform (DFT). Subsequently, the terminal equipment maps the obtained signal in the frequency region to a subcarrier and then restores the signal in a time region by inverse discrete Fourier transform (IDFT). Finally, the terminal equipment inserts a cyclic prefix (CP) in the signal restored in a time region and transmits the signal.
The base station removes the CP from the signal received from the terminal equipment, thereafter computes the value of crosscorrelation between the received signal and the pattern of the predetermined ZC sequence, and detects the ZC sequence transmitted from the terminal equipment and the cyclic shift of the ZC sequence based on the value of crosscorrelation.
The above-described technique has a problem described below.
It is desirable to simplify the configuration for detecting a pattern formed of a ZC sequence multiplexed by CDM and a cyclic shift and transmitted from a terminal equipment such as that described above. In doing so, a frequency region crosscorrelation method (multi-use channel estimation) is effective, in which a plurality of patterns having the same ZC sequence but having different cyclic shifts are detected in one circuit and the number of circuits is thereby reduced.
FIG. 2A is a block diagram showing a configuration for the base station in which a configuration for detecting patterns sent from terminal equipment is simplified. The configuration shown in FIG. 2 is made by considering channel estimation. FIG. 2B is a timing chart showing the result of channel estimation with the base station shown in FIG. 2A.
Referring to FIG. 2A, the received signal from which the CP has been removed is first converted into a frequency region by fast Fourier transform (FFT). Subsequently, the received signal in the frequency region and the conjugate complex number of a predetermined ZC sequence not cyclically shifted are multiplied together. Next, the signal obtained as the result of multiplication is converted into a time region by inverse fast Fourier transform (IFFT), thereby obtaining the value of crosscorrelation between the received signal and the predetermined ZC sequence. The ZC sequence transmitted from the terminal equipment is detected from the obtained crosscorrelation value.
A cyclic shift can be considered equivalent to a delay. From a crosscorrelation value delay profile shown in FIG. 2B by way of example, therefore, the amount of cyclic shift applied in each terminal equipment using the same ZC sequence can be detected. In the example shown in FIG. 2B, the peaks of crosscorrelation between terminal equipments UE0 to UE3 using different cyclic shifts with respect to the same ZC sequence appear at different times.
Use of this method makes it possible to obtain at one time the results of channel estimation on a plurality of terminal equipment that each have unique cyclic shifts with respect to the same ZC sequence.
Also, referring to FIG. 2A, not IDFT but IFFT is used to simplify the portion in which the value of crosscorrelation computed in a frequency region is converted into the value of crosscorrelation in a time region.
It is preferable that the number of samples be a prime number or a number having a large prime factor for compatibility with a sufficient number of ZC sequence differing in length.
If, in the base station, not IFFT such as that in FIG. 2A but IDFT corresponding to DFT in the terminal equipment is used, the number of samples for cyclic shift by the terminal equipment can be set in correspondence with an integral number of samples of the IDFT output in the base station. In such case, however, the configuration for inverse Fourier transform in the base station becomes complicated.
In the case shown in FIG. 2A, the configuration for inverse Fourier transform is simplified by using IFFT in the base station. However, this results in noncoincidence between the number of samples for cyclic shift by the terminal equipment and the integer number of samples of the IFFT output in the base station.
FIG. 3 is a diagram showing an example of multi-user channel estimation according to the mobile communication system having the terminal equipment shown in FIG. 1 and the base station shown in FIG. 2A. It is assumed that in this example terminal equipment UE0, UE1, UE2, and UE3 cyclically shift the same ZC sequence by amounts corresponding to 0, 18, 36 and 54 DFT samples, respectively, and that the DFT size (the number of samples) is 73 and each of the FFT size and the IFFT size is 256.
The rate of sampling in the IFFT output in the base station is 256/73 of the rate of sampling in the DFT input in terminal equipment. Accordingly, the amounts of cyclic shift by terminal equipment UE0, UE1, UE2, and UE3 correspond to 0, 63.12, 126.25 and 189.37 samples of the IFFT output in the base station. Values which are the results of subtraction of the integer portions from these values appear as timing errors in the results of channel estimation on terminal equipment UE. It can be understood that, as shown in FIG. 3, the results of channel estimation on terminal equipment UE other than terminal equipment UE0 contain timing errors smaller than the amount for 1 sample. More specifically, the result of channel estimation on UE1 contains a timing error corresponding to a 0.12 sample; the result of channel estimation on UE2 contains a timing error corresponding to a 0.25 sample; and the result of channel estimation on UE3 contains a timing error corresponding to a 0.37 sample.
In a case where these channel estimation results are used in demodulation in particular, timing errors act as a cause of a deterioration in demodulation performance. If compensation is made for such timing errors by signal processing in the frequency region, the configuration of the equipment becomes complicated.